System for Determining Nerve Direction to a Surgical Instrument

ABSTRACT

System for performing surgical procedures and assessments, including the use of neurophysiology-based monitoring to determine nerve proximity and nerve direction to surgical instruments employed in accessing a surgical target site.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/427,760 filed Mar. 22, 2012, now pending, which is a continuation ofU.S. patent application Ser. No. 11/182,545 filed Jul. 15, 2005 (nowU.S. Pat. No. 8,147,421), which is a continuation of InternationalApplication No. PCT/US03/02056 filed Jan. 15, 2003 and published on Aug.5, 2004 as International Publication No. WO 2004/064634.

BACKGROUND

Systems and methods exist for monitoring nerves and nerve muscles. Onesuch system determines when a needle is approaching a nerve. The systemapplies a current to the needle to evoke a muscular response. Themuscular response is visually monitored, typically as a shake or“twitch.” When such a muscular response is observed by the user, theneedle is considered to be near the nerve coupled to the responsivemuscle. These systems require the user to observe the muscular response(to determine that the needle has approached the nerve). This may bedifficult depending on the competing tasks of the user. In addition,when general anesthesia is used during a procedure, muscular responsemay be suppressed, limiting the ability of a user to detect theresponse.

While generally effective (although crude) in determining nerveproximity, such existing systems are incapable of determining thedirection of the nerve to the needle or instrument passing throughtissue or passing by the nerves. While the surgeon may appreciate that anerve is in the general proximity of the instrument, the inability todetermine the direction of the nerve relative to the instrument can leadto guess work by the surgeon in advancing the instrument, which raisesthe specter of inadvertent contact with, and possible damage to, thenerve.

SUMMARY

The present application may be directed to at least reduce the effectsof the above-described problems with the prior art. The presentapplication includes a system and related methods for determining thedirection of a surgical instrument to a nerve during surgicalprocedures. According to one aspect of the system, this involves the useof neurophysiology-based monitoring to determine nerve direction tosurgical instruments employed in accessing a surgical target site. Thesystem may do so in an automated, easy to use, and easy to interpretfashion so as to provide a surgeon-driven system.

The system may combine neurophysiology monitoring with any of a varietyof instruments used in or in accessing a surgical target site (referredto herein as “surgical access instruments”). By way of example only,such surgical access instruments may include, but are not necessarilylimited to, any number of devices or components for creating anoperative corridor to a surgical target site, such as K-wires,sequentially dilating cannula systems, distractor systems, and/orretractor systems. Although described herein largely in terms of use inspinal surgery, it is to be readily appreciated that the teachings ofthe methods and systems may be suitable for use in any number ofadditional surgical procedures where tissue having significant neuralstructures must be passed through (or near) in order to establish anoperative corridor to a surgical target site.

A general method according to the present application may include: (a)providing multiple (e.g., four orthogonally-disposed) electrodes aroundthe periphery of the surgical access instrument; (b) stimulating theelectrodes to identify the current threshold (I_(Thresh)) necessary toinnervate the muscle myotome coupled to the nerve near the surgicalaccess instrument; (c) determining the direction of the nerve relativeto the surgical access instrument via successive approximation; and (d)communicating this successive approximation direction information to thesurgeon in an easy-to-interpret fashion.

The act of providing multiple (e.g., four orthogonally-disposed)electrodes around the periphery of the surgical access instrument may beaccomplished in any number of suitable fashions depending upon thesurgical access instrument in question. For example, the electrodes maybe disposed orthogonally on any or all components of a sequentialdilation system (including an initial dilator, dilating cannulae, andworking cannula), as well as speculum-type and/or retractor-based accesssystems. The act of stimulating may be accomplished by applying any of avariety of suitable stimulation signals to the electrode(s) on thesurgical accessory, including voltage and/or current pulses of varyingmagnitude and/or frequency. The stimulating act may be performed duringand/or after the process of creating an operative corridor to thesurgical target site.

The act of determining the direction of the surgical access instrumentrelative to the nerve via successive approximation is preferablyperformed by monitoring or measuring the EMG responses of muscle groupsassociated with a particular nerve and innervated by the nerve(s)stimulated during the process of gaining surgical access to a desiredsurgical target site.

The act of communicating this successive approximation information tothe surgeon in an easy-to-interpret fashion may be accomplished in anynumber of suitable fashions, including but not limited to the use ofvisual indicia (such as alpha-numeric characters, light-emittingelements, and/or graphics) and audio communications (such as a speakerelement). By way of example only, this may include providing an arc orother graphical representation that indicates the general direction tothe nerve. The direction indicator may quickly start off relativelywide, become successively more narrow (based on improved accuracy overtime), and may conclude with a single arrow designating the relativedirection to the nerve.

Communicating this successive approximation information may be animportant feature. By providing such direction information, a user willbe kept informed as to whether a nerve is too close to a given surgicalaccessory element during and/or after the operative corridor isestablished to the surgical target site. This is particularlyadvantageous during the process of accessing the surgical target site inthat it allows the user to actively avoid nerves and redirect thesurgical access components to successfully create the operative corridorwithout impinging or otherwise compromising the nerves.

Based on this nerve direction feature, an instrument is capable ofpassing through virtually any tissue with minimal (if any) risk ofimpinging or otherwise damaging associated neural structures within thetissue, thereby making the system suitable for a wide variety ofsurgical applications.

A direction-finding algorithm that finds a stimulation threshold currentone electrode at a time for a plurality of electrodes (e.g., fourelectrodes) may require 40 to 80 stimulations in order to conclude witha direction vector. At a stimulation rate of 10 Hz, this method may takefour to eight seconds before any direction information is available to asurgeon. A surgeon may grow impatient with the system. An “arc” methoddescribed herein may improve the direction-finding algorithm and providenerve direction information to the surgeon sooner. The system maydisplay direction to the nerve during a sequence of stimulations as an“arc” (or wedge), which represents a zone containing the nerve.Computation of the direction arc (wedge) may be based on stimulationcurrent threshold ranges, instead of precise, finally-calculatedstimulation current threshold levels. Display of the direction arc(wedge) is possible at any time that the stimulation current thresholdsare known to fall within a range of values.

One aspect relates to a system comprising: a surgical accessory havingat least one stimulation electrode; and a control unit capable ofelectrically stimulating at least one stimulation electrode on saidsurgical accessory, sensing a response of a nerve depolarized by saidstimulation, determining a direction from the surgical accessory to thenerve based upon the sensed response, and communicating said directionto a user. The system may further comprise an electrode configured tosense a neuromuscular response of a muscle coupled to said depolarizednerve. The electrode may be operable to send the response to the controlunit.

The surgical accessory may comprise a system for establishing anoperative corridor to a surgical target site. The system forestablishing an operative corridor may comprise a series of sequentialdilation cannulae, where at least one cannula has said at least onestimulation electrode near a distal end. The system for establishing anoperative corridor may further comprise a K-wire. The K-wire may have afirst stimulation electrode at a distal tip. The K-wire may have asecond stimulation electrode away from the distal tip. The K-wire may beslidably received in the surgical accessory, where the surgicalaccessory has a plurality of electrodes. The system for establishing anoperative corridor may be configured to access a spinal target site. Thesystem for establishing an operative corridor may be configured toestablish an operative corridor via a lateral, trans-psoas approach.

The system may further comprise a handle coupled to the surgicalaccessory. The handle may have at least one button for initiating theelectrical stimulation from the control unit to at least one stimulationelectrode on the surgical accessory.

The control unit may comprise a display operable to display anelectromyographic (EMG) response of the muscle. The control unit maycomprise a touch-screen display operable to receive commands from auser.

The surgical accessory may comprise a plurality of stimulationelectrodes. The stimulation electrodes may be positioned near a distalend of the surgical accessory. The stimulation electrodes may bepositioned in a two-dimensional plane. The stimulation electrodes may bepositioned orthogonally to form a cross. The control unit may derive xand y Cartesian coordinates of a nerve direction with respect to thesurgical accessory by using x=i_(w) ²−i_(e) ² and y=i_(s) ²−i_(n) ²where i_(c), i_(w), i_(n), and i_(s) represent stimulation currentthresholds for east, west, north, and south electrodes. The stimulationelectrodes may comprise a first set of electrodes in a firsttwo-dimensional plane and a second set of at least one electrode inanother plane that is parallel to the first plane. The stimulationelectrodes may form a tetrahedron. The control unit may be configured todetermine a three-dimensional vector from a reference point on thesurgical accessory to a nerve.

The control unit may determine a three-dimensional vector from areference point on the surgical accessory to a nerve by using:

${x = {\frac{1}{4R}\left( {d_{w}^{2} - d_{e}^{2}} \right)}}\;$$y = {{\frac{1}{4R}\left( {d_{s}^{2} - d_{n}^{2}} \right)\mspace{14mu} {and}\mspace{14mu} z} = {\frac{1}{4D}{\left( {d_{o}^{2} - d_{k}^{2}} \right).}}}$

The control unit may be configured to determine a three-dimensionalvector from a reference point on the surgical accessory to a nerve byusing:

${x = {\frac{1}{4{RK}}\left( {i_{w} - i_{e}} \right)}}\;$$y = {{\frac{1}{4{RK}}\left( {i_{s} - i_{n}} \right)\mspace{14mu} {and}\mspace{14mu} z} = {\frac{1}{4{DK}}\left( {i_{c} - i_{k}} \right)}}$

where i_(x) is a stimulation current threshold of a correspondingstimulation electrode (west, east, south or north), i_(k) is thestimulation current threshold of a k-wire electrode, i_(s) is calculatedfrom: i_(c)+KR²=¼(i_(w)+i_(e)+i_(s)+i_(n)). The control unit may befurther configured to display the three-dimensional vector to a user.

The stimulation electrodes may comprise two pairs of electrodes. Thestimulation electrodes may comprise a first electrode at a firstlongitudinal level of the surgical accessory and a second electrode at asecond longitudinal level of the surgical accessory.

The control unit may be configured to electrically stimulate a firststimulation electrode with a first current signal, determine whether afirst stimulation current threshold has been bracketed, stimulate asecond stimulation electrode with a second current signal, and determinewhether a second stimulation current threshold has been bracketed. Thefirst and second current signals may be equal. The control unit may befurther configured to determine a first range for the first stimulationcurrent threshold, and determine a second range for the secondstimulation current threshold. Each range may have a maximum stimulationcurrent threshold value and a minimum stimulation current thresholdvalue. The control unit may be configured to process the first andsecond ranges by using x_(min)=i_(w,min) ²−i_(e,max) ²;x_(max)=i_(w,max) ²−i_(e,min) ²; y_(min)=i_(s,min) ²−i_(n,max) ²;y_(max)=i_(s,max) ²−i_(n,min) ², where i_(e), i_(w), i_(n), and i_(s)represent stimulation current thresholds for east, west, north, andsouth electrodes. The control unit may be configured to process thefirst and second ranges by using

${x_{\min} = {\frac{1}{4R}\left( {d_{w,\min}^{2} - d_{e,\max}^{2}} \right)}},{x_{\max} = {\frac{1}{4R}\left( {d_{w,\max}^{2} - d_{e,\min}^{2}} \right)}}$${y_{\min} = {\frac{1}{4R}\left( {d_{s,\min}^{2} - d_{n,\max}^{2}} \right)}},{y_{\max} = {\frac{1}{4R}\left( {d_{s,\max}^{2} - d_{n,\min}^{2}} \right)}}$

where d is a distance from a nerve to east, west, north, and southelectrodes.

The control unit may be configured to process the first and secondranges and display an arc indicating a general direction of a nerve fromthe surgical accessory. The control unit may be further configured toelectrically stimulate the first stimulation electrode with a thirdcurrent signal, determine whether the first stimulation currentthreshold has been bracketed, stimulate the second stimulation electrodewith a fourth current signal, and determine whether the secondstimulation current threshold has been bracketed.

The control unit may be configured to electrically stimulate eachelectrode until a stimulation current threshold has been bracketed. Thecontrol unit may be configured to display an arc indicating a generaldirection of a nerve from the surgical accessory and narrow the arc asstimulation current thresholds are bracketed. The control unit may befurther configured to electrically stimulate the first and secondstimulation electrodes to bisect each bracket until a first stimulationcurrent threshold has been found for the first stimulation electrode anda second stimulation current threshold has been found for the secondstimulation electrode within a predetermined range of accuracy. Thecontrol unit may be configured to display an arc indicating a generaldirection of a nerve from the surgical accessory and narrow the arc asstimulation current threshold brackets are bisected.

The control unit may be configured to emit a sound when the control unitdetermines a distance between the surgical accessory and the nerve hasreached a predetermined level. The control unit may be configured toemit a sound that indicates a distance between the surgical accessoryand the nerve. The surgical accessory may be dimensioned to be insertedpercutaneously through a hole to a surgical site.

Another aspect relates to a surgical instrument comprising an elongatedbody and a plurality of electrodes on the elongated body. Each electrodeis configured to produce electrical current pulses at a plurality ofcurrent levels. At least one current level being sufficient todepolarize a nerve when the elongated body is near the nerve. Theelongated body may comprise a K-wire. The elongated body may comprise acannula. The electrodes may comprise four orthogonal electrodes in atwo-dimensional plane. The electrodes may comprise a first set ofelectrodes in a first two-dimensional plane and a second set of at leastone electrode in another plane that is parallel to the first plane. Theelectrodes may be configured to produce electrical current pulsesround-robin at a first current level, then produce electrical currentpulses round-robin at a second current level. The elongated body maycomprise a sequential dilation system.

Another aspect relates to a processing unit operable to determine rangesof nerve-stimulation threshold current levels for a plurality ofelectrodes on a surgical instrument inserted into a body.

Another aspect relates to a method comprising providing a systemoperable to determine a direction of a nerve from a surgical instrumentinserted in a body; and installing software in the system. The methodmay further comprise configuring a minimum threshold peak-to-peakvoltage level of a neuromuscular response.

Another aspect relates to a method of finding a direction of a nervefrom a surgical instrument. The method comprises: electricallystimulating a first stimulation electrode on a surgical instrumentinserted in a body with a first current signal; determining whether afirst stimulation current threshold has been bracketed by the firststimulation current signal; electrically stimulating a secondstimulation electrode on the surgical instrument with a second currentsignal; and determining whether a second stimulation current thresholdhas been bracketed by the second stimulation current signal.

The first and second current signals may be equal. The method mayfurther comprise determining a first range for the first stimulationcurrent threshold, and determining a second range for the secondstimulation current threshold, each range having a maximum stimulationcurrent threshold value and a minimum stimulation current thresholdvalue. The method may further comprise processing the first and secondranges and displaying an arc indicating a general direction of a nervefrom the surgical accessory.

The method may further comprise electrically stimulating the firststimulation electrode with a third current signal; determining whetherthe first stimulation current threshold has been bracketed; stimulatingthe second stimulation electrode with a fourth current signal; anddetermining whether the second stimulation current threshold has beenbracketed. The method may further comprise electrically stimulating eachelectrode until a stimulation current threshold has been bracketed. Themethod may further comprise displaying an arc indicating a generaldirection of a nerve from the surgical accessory and narrowing the arcas stimulation current thresholds are bracketed.

The method may further comprise electrically stimulating the first andsecond stimulation electrodes to bisect each bracket until a firststimulation current threshold has been found for the first stimulationelectrode and a second stimulation current threshold has been found forthe second stimulation electrode within a predetermined range ofaccuracy. The method may further comprise displaying an arc indicating ageneral direction of a nerve from the surgical accessory and narrow thearc as stimulation current threshold brackets are bisected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating fundamental functions of aneurophysiology-based surgical system according to one embodiment of thepresent application;

FIG. 2 is a perspective view of an exemplary surgical system capable ofperforming the functions in FIG. 1 and determining nerve direction tosurgical instruments employed in accessing a surgical target site;

FIG. 3 is a block diagram of the surgical system shown in FIG. 2;

FIG. 4 is a graph illustrating a plot of a stimulation current pulsecapable of producing a neuromuscular response (EMG) of the type shown inFIG. 5;

FIG. 5 is a graph illustrating a plot of the neuromuscular response(EMG) of a given myotome over time based on a current stimulation pulse(such as shown in FIG. 4) applied to a nerve bundle coupled to the givenmyotome;

FIG. 6 is a graph illustrating a plot of EMG response peak-to-peakvoltage (Vpp) for each given stimulation current level (Isom) forming astimulation current pulse (otherwise known as a “recruitment curve”) forthe system of FIG. 2;

FIG. 7A-7E are graphs illustrating a current threshold-hunting algorithmthat may be used by the system of FIG. 2;

FIG. 8 illustrates four orthogonal electrodes near a distal end of asurgical instrument, such as a cannula, modeled as north, south, eastand west points in a two-dimensional X-Y plane for the system of FIG. 2;

FIG. 9 illustrates a nerve point (x, y) bounded by maximum and minimumx- and y-values, which forms a rectangle;

FIG. 10 illustrates a vector from an origin (center axis of aninstrument with electrodes) to a nerve point (x, y) and an arccontaining that vector found by the system of FIG. 2;

FIG. 11 illustrates a stimulation site and multiple EMG response sensingsites for the system of FIG. 2;

FIG. 12 is a graph illustrating a plot of a neuromuscular response (EMG)over time in response to a stimulus current pulse, where the plot showsvoltage extrema at times T1 and T2;

FIG. 13 is a graph illustrating a method of determining the direction ofa nerve (denoted as a “hexagon”) relative to an instrument having four(4) orthogonally disposed stimulation electrodes (denoted by the“circles”) for the system of FIG. 2;

FIG. 14A is a side view and FIG. 14B is a front view of a distal end ofa surgical instrument, such as a cannula in FIG. 2, with four orthogonalelectrodes and a fifth electrode;

FIG. 15A-15C are displays of a surgical instrument in FIG. 2 and a nervedirection arc that may become progressively smaller until it becomes anarrow as stimulation threshold levels are bracketed, bisected and foundfor a plurality of electrodes in FIGS. 14A-14B;

FIGS. 16-19 illustrate a sequential dilation access system of FIG. 2 inuse creating an operative corridor to an intervertebral disk;

FIGS. 20-21 are exemplary screen displays illustrating one embodiment ofthe nerve direction feature of the surgical access system of FIG. 2;

FIG. 22 illustrates a generalized one-dimensional, two-electrode,direction-finding model;

FIG. 23 illustrates an electrode positioned along the x=y=0 z-axis,which is in a different x-y plane than a plurality of other electrodes;

FIG. 24 illustrates a first pair of electrodes in one plane, a secondpair of electrodes in another plane and a nerve activation site;

FIG. 25 illustrates a device with four electrodes in a tetrahedronconfiguration;

FIG. 26 illustrates a device and a K-wire slidably received in thedevice with electrodes.

DETAILED DESCRIPTION

Illustrative embodiments of the application are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

FIG. 1 illustrates general functions according to one embodiment of thepresent application, namely: (a) providing multiple (e.g., fourorthogonally-disposed) electrodes around the periphery of the surgicalaccess instrument; (b) stimulating the electrodes to identify thecurrent threshold (I_(Thresh)) necessary to innervate the muscle myotomecoupled to the nerve near the surgical access instrument; (c)determining the direction of the nerve relative to the surgical accessinstrument via successive approximation; and (d) communicating thissuccessive approximation direction information to the surgeon in aneasy-to-interpret fashion.

The act of providing multiple (e.g., four orthogonally-disposed)electrodes around the periphery of the surgical access instrument may beaccomplished in any number of suitable fashions depending upon thesurgical access instrument in question. For example, the electrodes maybe disposed orthogonally on any or all components of a sequentialdilation system (including an initial dilator, dilating cannulae, andworking cannula), as well as speculum-type and/or retractor-based accesssystems, as disclosed in the co-pending, co-assigned May, 2002 U.S.Provisional application incorporated above. The act of stimulating maybe accomplished by applying any of a variety of suitable stimulationsignals to the electrode(s) on the surgical accessory, including voltageand/or current pulses of varying magnitude and/or frequency. Thestimulating act may be performed during and/or after the process ofcreating an operative corridor to the surgical target site.

The act of determining the direction of the surgical access instrumentrelative to the nerve via successive approximation is preferablyperformed by monitoring or measuring the EMG responses of muscle groupsassociated with a particular nerve and innervated by the nerve(s)stimulated during the process of gaining surgical access to a desiredsurgical target site.

The act of communicating this successive approximation information tothe surgeon in an easy-to-interpret fashion may be accomplished in anynumber of suitable fashions, including but not limited to the use ofvisual indicia (such as alpha-numeric characters, light-emittingelements, and/or graphics) and audio communications (such as a speakerelement). By way of example only, this may include providing an arc orother graphical representation that indicates the general direction tothe nerve. The direction indicator may quickly start off relativelywide, become successively more narrow (based on improved accuracy overtime), and may conclude with a single arrow designating the relativedirection to the nerve.

The direction indicator may be an important feature. By providing suchdirection information, a user will be kept informed as to whether anerve is too close to a given surgical accessory element during and/orafter the operative corridor is established to the surgical target site.This is particularly advantageous during the process of accessing thesurgical target site in that it allows the user to actively avoid nervesand redirect the surgical access components to successfully create theoperative corridor without impinging or otherwise compromising thenerves.

Based on this nerve direction feature, then, an instrument is capable ofpassing through virtually any tissue with minimal (if any) risk ofimpinging or otherwise damaging associated neural structures within thetissue, thereby making the system suitable for a wide variety ofsurgical applications.

FIGS. 2-3 illustrate, by way of example only, a surgical system 20provided in accordance with a broad aspect of the present application.The surgical system 20 includes a control unit 22, a patient module 24,an EMG harness 26 and return electrode 28 coupled to the patient module24, and (by way of example only) a sequential dilation surgical accesssystem 34 capable of being coupled to the patient module 24 via cable32. The sequential dilation access system 34 comprises, by way ofexample only, a K-wire 46, one or more dilating cannula 48, and aworking cannula 50.

The control unit 22 includes a touch screen display 40 and a base 42,which collectively contain the essential processing capabilities(software and/or hardware) for controlling the surgical system 20. Thecontrol unit 22 may include an audio unit 18 that emits sounds accordingto a location of a surgical element with respect to a nerve, asdescribed herein.

The patient module 24 is connected to the control unit 22 via a datacable 44, which establishes the electrical connections andcommunications (digital and/or analog) between the control unit 22 andpatient module 24. The main functions of the control unit 22 includereceiving user commands via the touch screen display 40, activatingstimulation electrodes on the surgical access instruments 30, processingsignal data according to defined algorithms (described below),displaying received parameters and processed data, and monitoring systemstatus and report fault conditions. The touch screen display 40 ispreferably equipped with a graphical user interface (GUI) capable ofcommunicating information to the user and receiving instructions fromthe user. The display 40 and/or base 42 may contain patient moduleinterface circuitry (hardware and/or software) that commands thestimulation sources, receives digitized signals and other informationfrom the patient module 24, processes the EMG responses to extractcharacteristic information for each muscle group, and displays theprocessed data to the operator via the display 40.

In one embodiment, the surgical system 20 is capable of determiningnerve direction relative to each K-wire 46, dilation cannula 48 and/orthe working cannula 50 during and/or following the creation of anoperative corridor to a surgical target site. Surgical system 20accomplishes this by having the control unit 22 and patient module 24cooperate to send electrical stimulation signals to each of theorthogonally-disposed stimulation electrodes 1402A-1402D (FIGS. 14A-14B)on the various surgical access instruments 46-50 (e.g., electrodes onthe distal ends of the instruments 46-50). Depending upon the locationof the surgical access instruments 46-50 within a patient, thestimulation signals may cause nerves adjacent to or in the generalproximity of the surgical instruments 46-50 to depolarize. This causesmuscle groups to innervate and generate EMG responses, which can besensed via the EMG harness 26. The nerve direction feature of the system20 is based on assessing the evoked response of the various musclemyotomes monitored by the surgical system 20 via the EMG harness 26.

The sequential dilation surgical access system 34 is designed to bluntlydissect the tissue between the patient's skin and the surgical targetsite. Each K-wire 46, dilating cannula 48 and/or working cannula 50 maybe equipped with multiple (e.g., four orthogonally-disposed) stimulationelectrodes to detect the location of nerves in between the skin of thepatient and the surgical target site. To facilitate this, a surgicalhand-piece 52 is provided for electrically coupling the surgicalaccessories 46-50 to the patient module 24 (via cable 32). In apreferred embodiment, the surgical hand piece 52 includes one or morebuttons for selectively initiating the stimulation signal (preferably, acurrent signal) from the control unit 22 to a particular surgical accessinstrument 46-50. Stimulating the electrode(s) on these surgical accessinstruments 46-50 during passage through tissue in forming the operativecorridor will cause nerves that come into close or relative proximity tothe surgical access instruments 46-50 to depolarize, producing aresponse in the innervated myotome.

By monitoring the myotomes associated with the nerves (via the EMGharness 26 and recording electrode 27) and assessing the resulting EMGresponses (via the control unit 22), the sequential dilation accesssystem 34 is capable of detecting the direction to such nerves.Direction determination provides the ability to actively negotiatearound or past such nerves to safely and reproducibly form the operativecorridor to a particular surgical target site. In one embodiment, by wayof example only, the sequential dilation access system 34 isparticularly suited for establishing an operative corridor to anintervertebral target site in a postero-lateral, trans-psoas fashion soas to avoid the bony posterior elements of the spinal column.

A discussion of the algorithms and principles behind the neurophysiologyfor accomplishing these functions will now be undertaken, followed by adetailed description of the various implementations of these principles.

FIGS. 4 and 5 illustrate a fundamental aspect of the presentapplication: a stimulation signal (FIG. 4) and a resulting evokedresponse (FIG. 5). By way of example only, the stimulation signal ispreferably a stimulation current signal (I_(Stim)) having rectangularmonophasic pulses with a frequency and amplitude adjustable by systemsoftware. In one embodiment, the stimulation current (I_(Stim)) may becoupled in any suitable fashion (i.e., AC or DC) and comprisesrectangular monophasic pulses of 200 microsecond duration. The amplitudeof the current pulses may be fixed, but may preferably sweep fromcurrent amplitudes of any suitable range, such as from 2 to 100 mA. Foreach nerve and myotome there is a characteristic delay from thestimulation current pulse to the EMG response (typically between 5 to 20ms). To account for this, the frequency of the current pulses may be setat a suitable level, such as, in a preferred embodiment, 4 Hz to 10 Hz(and most preferably 4.5 Hz), so as to prevent stimulating the nervebefore it has a chance to recover from depolarization. The EMG responseshown in FIG. 5 can be characterized by a peak-to-peak voltage ofV_(pp)=V_(max)−V_(min).

A basic premise behind the neurophysiology employed by the system 20 isthat each nerve has a characteristic threshold current level(I_(Thresh)) at which it will depolarize. Below this threshold, currentstimulation will not evoke a significant EMG response (V_(pp)). Once thestimulation threshold (I_(Thresh)) is reached, the evoked response isreproducible and increases with increasing stimulation until saturationis reached. This relationship between stimulation current and EMGresponse may be represented graphically via a so-called “recruitmentcurve,” such as shown in FIG. 6, which includes an onset region, alinear region, and a saturation region. By way of example only, thesystem 20 may define a significant EMG response to have a Vpp ofapproximately 100 uV. In a preferred embodiment, the lowest stimulationcurrent that evokes this threshold voltage (V_(thresh)) is called astimulation current threshold or “I_(Thresh).”

In order to obtain this useful information, the system 20 should firstidentify the peak-to-peak voltage (Vpp) of each EMG response thatcorresponds to a given stimulation current (I_(Stim)). The existence ofstimulation and/or noise artifacts, however, can conspire to create anerroneous Vpp measurement of the electrically evoked EMG response. Toovercome this challenge, the surgical system 20 may employ any number ofsuitable artifact rejection techniques. Having measured each Vpp EMGresponse (as facilitated by the stimulation and/or noise artifactrejection techniques), this Vpp information is then analyzed relative tothe stimulation current in order to determine a relationship between thenerve and the given electrode on the surgical access instrument 46-50transmitting the stimulation current. More specifically, the system 20determines these relationships (between nerve and surgical accessory) byidentifying the minimum stimulation current (I_(Thresh)) capable ofproducing a predetermined Vpp EMG response.

I_(Thresh) may be determined for each of the four orthogonal electrodes1402A-1402D (FIGS. 14A-14B) in an effort to determine the directionbetween the surgical access instrument 34, 36 and the nerve. This may beaccomplished by employing a two-part threshold-hunting algorithm,including a bracketing process and a bi-section (or bisecting) process,which may proceed step-wise for each stimulation electrode to providesuccessive directional information to the user.

Arc Method

In one embodiment, successive directional information may take the formof an arc or wedge (or range) representing a zone that contains thenerve, according to an “arc” method described below. This successivedirectional information is based on stimulation current threshold“ranges,” and may be displayed (FIGS. 15A-15C) or otherwise communicatedto the surgeon any time the stimulation current thresholds are known tofall within a range of values.

In the bracketing process, an electrical stimulus is provided at each ofthe four orthogonal electrodes 1402A-1402D (FIGS. 14A-14B), beginningwith a small current level (e.g. 0.2 mA) and ramping up. In the “arc”method, each of the four electrodes 1402A-1402D may be stimulated at thesame current level, in sequence, before proceeding to the next highercurrent level (as opposed to another method that completes thebracketing for one electrode 1402 before advancing to another electrode1402). The goal is to identify a bracket around the stimulation currentfor each of the four stimulation electrodes 1402A-1402D. If astimulation current threshold has been bracketed for an electrode 1402,the bracketing act is complete for that electrode 1402, and stimulationproceeds for the remaining electrodes until the stimulation currentthreshold has been bracketed for each electrode. As the bracketingprocess proceeds, each new stimulation provides information about the“range” of the current threshold for that electrode 1402. Thisinformation may bracket the stimulation current threshold (e.g., between1.6 and 3.2 mA), or it may only provide a lower bound for the currentthreshold (e.g., threshold is greater than 5.0 mA). In either event, the“arc” bracketing process proceeds for each of the stimulation electrodes1402A-1402D to provide, in succession, more accurate informationregarding the direction of the nerve relative to the surgical accessinstrument 46-50.

As shown in FIG. 10, an arc (wedge) containing the final directionvector is computed from the range information for the stimulationcurrent thresholds corresponding to the four stimulation electrodes1402A-1402D. This can be done as often as desired as the bracketingmethod proceeds. The arc (wedge) may then be used to display directionalinformation to the operator, as in FIGS. 15A-15C.

This successive approximation information may be communicated to thesurgeon in a number of easy-to-interpret fashions, including but notlimited to the use of visual indicia (such as alpha-numeric characters,light-emitting elements, and/or graphics, as in FIGS. 15A-15C and 20-21)and audio communications (such as a speaker element 18 in FIG. 3). Byway of example only, this successive directional information may includeproviding an “arc” 1502 (hence the name “arc” method) or other graphicalrepresentation that indicates the general direction to the nerve, whichmay start off relatively wide, become successively more narrow (based onimproved accuracy over time), and may conclude with a single arrowdesignating the relative direction to the nerve. FIGS. 15A-15Cillustrate screenshots of a cross-section of an instrument 1500 and awide direction arc 1502A, a narrower direction arc 1502B and an arrow1502C as more stimulation current pulses are generated and EMG responsesare analyzed during the bracketing and bisecting processes.

There are a number of possibilities for displaying the arc information.An arc or wedge might be displayed. Alternatively, an arrow might pointto the midpoint of the arc. Another indicator might be used toillustrate the width of the arc (i.e. the uncertainty remaining in theresult).

Upon completion of the bracketing process, a bisection process maydetermine more precisely the stimulation current thresholds. As with thebracketing process, current stimulations may be “rotated” among thestimulation electrodes so that the thresholds are refined substantiallyin parallel, according to the “arc” method. As with the bracketingmethod, the arc (wedge) 1502 containing the final direction vector maybe computed and displayed (FIGS. 15A-15C) frequently during the process.Upon completion of the bisection method for all electrodes 1402A-1402D,the stimulation current thresholds are identified precisely. At thattime, the final direction vector 1502C (FIG. 15C and FIGS. 20-21) may bedisplayed.

The above-identified two-part hunting-algorithm may be further explainedwith reference to FIGS. 7A-7E. According to the arc method, eachelectrode 1402 is stimulated at the same stimulation current levelbefore passing to the next stimulation current level. In this fashion,successive directional information can be obtained as described above.Threshold current (I_(Thresh)) is the minimum stimulation current(I_(Stim)) (FIG. 6) that produces a Vpp (FIG. 5) greater than a knownthreshold voltage (V_(Thresh)). The value of I_(Stim) may be adjusted bya bracketing method as follows. The first bracket may be 0.2 mA and 0.3mA. If the Vpp corresponding to both of these stimulation currents islower than Vthresh, then the bracket size may be doubled to 0.2 mA and0.4 mA. This doubling of the bracket size continues until the upper endof the bracket results in a Vpp that is above V_(Thresh).

The size of the brackets may then be reduced by a bisection method. Acurrent stimulation value at the midpoint of the bracket is used, and ifthis results in a Vpp that is above Vthresh, then the lower half becomesthe new bracket. Likewise, if the midpoint Vpp is below Vthresh, thenthe upper half becomes the new bracket. This bisection method is useduntil the bracket size has been reduced to I_(Thresh) mA. I_(Thresh) maybe selected as a value falling within the bracket, but is preferablydefined as the midpoint of the bracket.

The threshold-hunting algorithm of this embodiment may support threestates: bracketing, bisection, and monitoring. A “stimulation currentbracket” is a range of stimulation currents that bracket the stimulationcurrent threshold I_(Thresh). The width of a bracket is the upperboundary value minus the lower boundary value. If the stimulationcurrent threshold I_(Thresh) of a channel exceeds the maximumstimulation current, that threshold is considered out-of-range. Duringthe bracketing state, threshold hunting will employ the method describedherein to select stimulation currents and identify stimulation currentbrackets for each EMG channel in range.

The initial bracketing range may be provided in any number of suitableranges. In one embodiment, the initial bracketing range is 0.2 to 0.3mA. If the upper stimulation current does not evoke a response, theupper end of the range should be increased. For example, the range scalefactor may be 2. The stimulation current should preferably not beincreased by more than 10 mA in one iteration. The stimulation currentshould preferably never exceed a programmed maximum stimulation current(to prevent nerve damage, injury or other undesirable effects). For eachstimulation, the algorithm will examine the response of each activechannel to determine whether the stimulation current falls within thatbracket. Once the stimulation current threshold of each channel has beenbracketed, the algorithm transitions to the bisection state.

During the bisection state (FIGS. 7C and 7D), threshold hunting mayselect stimulation currents and narrow the bracket to a selected width(for example, 0.1 mA) for each EMG channel with an in-range threshold.After the minimum stimulation current has been bracketed (FIG. 7B), therange containing the root is refined until the root is known with aspecified accuracy. The bisection method is used to refine the rangecontaining the root. In one embodiment, the root should be found to aprecision of 0.1 mA. During the bisection method, the stimulationcurrent at the midpoint of the bracket is used. If the stimulationevokes a response, the bracket shrinks to the lower half of the previousrange. If the stimulation fails to evoke a response, the bracket shrinksto the upper half of the previous range. The nerve proximity/directiondetection algorithm is locked on the electrode position when theresponse threshold is bracketed by stimulation currents separated by theselected width (i.e., 0.1 mA). The process is repeated for each of theactive channels until all thresholds are precisely known. At that time,the algorithm may enter the monitoring state.

During the monitoring state (FIG. 7E), threshold hunting may employ themethod described below to select stimulation currents and identifywhether stimulation current thresholds are changing. In the monitoringstate, the stimulation current level may be decremented or incrementedby 0.1 mA, depending on the response of a specific channel. If thethreshold has not changed, then the lower end of the bracket should notevoke a response, while the upper end of the bracket should. If eitherof these conditions fail, the bracket is adjusted accordingly. Theprocess is repeated for each of the active channels to continue toassure that each threshold is bracketed. If stimulations fail to evokethe expected response three times in a row, then the algorithm maytransition back to the bracketing state in order to reestablish thebracket.

A method for computing the successive arc/wedge directional informationfrom stimulation current threshold range information is described. Thestimulation current threshold is presumed to be proportional to adistance to the nerve. The nerve may be modeled as a single point. Sincestimulation current electrodes are in an orthogonal array, calculationof the X- and Y-dimension components of the direction vector may proceedindependently. With reference to FIG. 8, the North and South electrodes800A, 800C contribute to the Y-dimension component, while the East andWest electrodes 800B-800D contribute to the X-dimension component. Thedirection vector <x, y> to a nerve may be defined as:

x=i _(w) ² −i _(e) ²

y=i _(s) ² −i _(n) ²  (1)

where i_(c), i_(w), i_(n), and i_(s) represent the stimulation currentthresholds for the east, west, north, and south electrodes 802B, 802D,802A, 802C, respectively. (The equations may be normalized to anarbitrary scale for convenience.)

As the threshold hunting method begins, the stimulation currentthresholds are known only within a range of values. Therefore, the X-and Y-dimension components are known only within a range. This methodprovides an extension to the previous definitions, as follows:

x _(min) =i _(w,min) ² −i _(e,max) ²

x _(max) =i _(w,max) ² −i _(e,min) ²

y _(min) =i _(s,min) ² −i _(n,max) ²

y _(max) =i _(s,max) ² −i _(n,min) ²  (2)

Just as i_(e,min) and i_(e,max) bracket i_(e), x and y are bracketed by[x_(min), x_(max)] and [y_(min), y_(max)]. Stated another way, the point(x,y) lies within a rectangle 900 described by these boundaries, asshown in FIG. 9. As shown in FIG. 10, just as the point (x,y) representsa vector from the origin to a nerve modeled as a point, the boundingrectangle 900 represents an arc (wedge) containing that vector.

The arc method may have several advantages. First, the arc is capable ofnarrowing in a relatively quick fashion as more stimulations andresponses are analyzed. This method provides general directionalinformation much faster than if the current threshold for each electrode1402 was determined before moving on the next electrode 1402. Withgeneral directional information, it may be possible to terminate thestimulation before having the ultimate precision of the stimulationcurrent vectors. This will result in a faster response, in manyinstances. The arc method may provide a real-time view of thedata-analysis. This helps illustrate the value of additionalstimulations to a user. This educates and empowers the user. The usercan observe the progress of this method, which aids in the understandingof the time the system 20 takes to converge on the final directionvector. More frequent display updates help time “go faster” for theuser. This method avoids a long pause that might seem even longer.Disclosure of the intermediate acts (narrowing arcs) in the process offinding the direction vector invites mutual trust between the user andthe access system 30. An arc may provide a more intuitive visualizationfor neural tissue than a direction vector.

The arc method may also be useful in tracking direction as theinstrument and stimulation electrodes move relative to the nerve. Forexample, if the uncertainty in the stimulation current thresholdincreases, this can be reflected in an increasing arc size.

The sequential dilation access system 34 (FIG. 2) of the system 20 iscapable of accomplishing safe and reproducible access to a surgicaltarget site. It does so by detecting the existence of and direction toneural structures before, during, and after the establishment of anoperative corridor through (or near) any of a variety of tissues havingsuch neural structures. If neural structures are contacted or impinged,this may result in neural impairment for the patient.

In one embodiment, the surgical system 20 accomplishes this through theuse of the surgical hand-piece 52, which may be electrically coupled tothe K-wire 46 via a first cable connector 51 a, 51 b and to either thedilating cannula 48 or the working cannula 50 via a second cableconnector 53 a, 53 b. For the K-wire 46 and working cannula 50, cablesare directly connected between these accessories and the respectivecable connectors 51 a, 53 a for establishing electrical connection tothe stimulation electrode(s). In one embodiment, a pincher or clamp-typedevice 57 is provided to selectively establish electrical communicationbetween the surgical hand-piece 52 and the stimulation electrode(s) onthe distal end of the cannula 48. This is accomplished by providingelectrical contacts on the inner surface of the opposing arms formingthe clamp-type device 57, wherein the contacts are dimensioned to beengaged with electrical contacts (preferably in a male-female engagementscenario) provided on the dilating cannula 48 and working cannula 50.The surgical hand-piece 52 includes one or more buttons such that a usermay selectively direct a stimulation current signal from the controlunit 22 to the electrode(s) on the distal ends of the surgical accesscomponents 46-50. In an important aspect, each surgical access component46-50 is insulated along its entire length, with the exception of theelectrode(s) at their distal end. In the case of the dilating cannula 48and working cannula 50, the electrical contacts at their proximal endsfor engagement with the clamp 57 are not insulated. The EMG responsescorresponding to such stimulation may be monitored and assessed in orderto provide nerve proximity and/or nerve direction information to theuser.

When employed in spinal procedures, for example, such EMG monitoringwould preferably be accomplished by connecting the EMG harness 26 to themyotomes in the patient's legs corresponding to the exiting nerve rootsassociated with the particular spinal operation level (see FIGS. 11 and20-21). In a preferred embodiment, this is accomplished via 8 pairs ofEMG electrodes 27 (FIG. 2) placed on the skin over the major musclegroups on the legs (four per side), an anode electrode 29 providing areturn path for the stimulation current, and a common electrode 31providing a ground reference to pre-amplifiers in the patient module 24.Although not shown, it will be appreciated that any of a variety ofelectrodes can be employed, including but not limited to needleelectrodes. The EMG responses measured via the EMG harness 26 provide aquantitative measure of the nerve depolarization caused by theelectrical stimulus. By way of example, the placement of EMG electrodes27 may be undertaken according to the manner shown in Table 1 below forspinal surgery:

TABLE 1 Color Channel ID Myotome Spinal Level Blue Right 1 Right VastusMedialis L2, L3, L4 Violet Right 2 Right Tibialis Anterior L4, L5 GreyRight 3 Right Biceps Femoris L5, S1, S2 White Right 4 Right Gastroc.Medial S1, S2 Red Left 1 Left Vastus Medialis L2, L3, L4 Orange Left 2Left Tibialis Anterior L4, L5 Yellow Left 3 Left Biceps Femoris L5, S1,S2 Green Left 4 Left Gastroc. Medial S1, S2

FIGS. 16-19 illustrate the sequential dilation access system 34 in FIG.2 in use creating an operative corridor to an intervertebral disk. Asshown in FIG. 16, an initial dilating cannula 48A is advanced towardsthe target site with the K-wire 46 disposed within an inner lumen withinthe dilating cannula 48. This may be facilitated by first aligning theK-wire 46 and initial dilating cannula 48A using any number ofcommercially available surgical guide frames. In one embodiment, as bestshown in the expanded insets A and B, the K-wire 46 and initial dilatingcannula 48A are each equipped with a single stimulation electrode 70 todetect the presence and/or location of nerves in between the skin of thepatient and the surgical target site. More specifically, each electrode70 may be positioned at an angle relative to the longitudinal axis ofthe K-wire 46 and dilator 48 (and working cannula 50). In oneembodiment, this angle may range from 5 to 85 degrees from thelongitudinal axis of these surgical access components 46-50. Byproviding each stimulation electrode 70 in this fashion, the stimulationcurrent will be directed angularly from the distal tip of the respectiveaccessory 46, 48. This electrode configuration is advantageous indetermining proximity, as well as direction, according to the presentapplication in that a user may simply rotate the K-wire 46 and/ordilating cannula 48 while stimulating the electrode 70. This may be donecontinuously or step-wise, and preferably while in a fixed axialposition. In either case, the user will be able to determine thelocation of nerves by viewing the proximity information on the displayscreen 40 and observing changes as the electrode 70 is rotated. This maybe facilitated by placing a reference mark 72 on the K-wire 46 and/ordilator 48 (or a control element coupled thereto), indicating theorientation of the electrode 70 to the user.

In another embodiment, the K-wire 46 and dilating cannula 48 in FIG. 2may each have multiple electrodes, as described above and shown in FIGS.14A-14B.

In the embodiment shown, the trajectory of the K-wire 46 and initialdilator 48A is such that they progress towards an intervertebral targetsite in a postero-lateral, trans-psoas fashion so as to avoid the bonyposterior elements of the spinal column. Once the K-wire 46 is dockedagainst the annulus of the particular intervertebral disk, cannulae ofincreasing diameter 48B-48D may then be guided over the previouslyinstalled cannula 48A (sequential dilation) until a desired lumendiameter is installed, as shown in FIG. 17. By way of example only, thedilating cannulae 48A-48D may range in diameter from 6 mm to 30 mm, withlength generally decreasing with increasing diameter size. Depth indicia72 may be optionally provided along the length of each dilating cannula48 to aid the user in gauging the depth between the skin of the patientand the surgical target site. As shown in FIG. 18, the working cannula50 may be slideably advanced over the last dilating cannula 48D after adesired level of tissue dilation has been achieved. As shown in FIG. 19,the last dilating cannula 48D and then all the dilating cannulae 48 maythen be removed from inside the inner lumen of the working cannula 50 toestablish the operative corridor therethrough.

Once a nerve is detected using the K-wire 46, dilating cannula 48, orthe working cannula 50, the surgeon may select the DIRECTION function todetermine the angular direction to the nerve relative to a referencemark on the access components 46-50, as shown in FIG. 21. In oneembodiment, a directional arrow 90 is provided, by way of example only,disposed around the cannula graphic 87 for the purpose of graphicallyindicating to the user which direction the nerve is relative to theaccess components 46-50. This information helps the surgeon avoid thenerve as he or she advances the K-wire 46 and cannulae 48, 50. In oneembodiment, this directional capability is accomplished by equipping theK-wire 46, dilators 48 and working cannula 50 with four (4) stimulationelectrodes disposed orthogonally on their distal tip (FIGS. 14A-14B).These electrodes are preferably scanned in a monopolar configuration(that is, using each of the 4 electrodes as the stimulation source). Thethreshold current (I_(thresh)) is found for each of the electrodes bymeasuring the muscle evoked potential response Vpp and comparing it to aknown threshold Vthresh. From this information, the direction from astimulation electrode (or device 46-50) to a nerve may be determinedaccording to the algorithm and technique described herein and withreference to FIGS. 8-10, 13, 14A-14B and 15A-15C. In FIGS. 8 and 13, thefour (4) electrodes 800A-800D are placed on the x and y axes of a twodimensional coordinate system at a radius R from the origin. A vector isdrawn from the origin along the axis corresponding to each electrode.Each vector has a length equal to I_(Thresh) for that electrode. Thus,with four electrodes 800A-800D, four vectors are drawn from the originalong the four axi corresponding to the four electrodes. The vector fromthe origin to a direction pointing toward the nerve is then computed.Using the geometry shown, the (x,y) coordinates of the nerve, taken as asingle point, can be determined as a function of the distance from thenerve to each of four electrodes 800A-800D. This can be expresslymathematically as follows:

-   -   Where the “circles” in FIG. 13 denote the position of the        electrode respective to the origin or center of the cannula, the        “hexagon” denotes the position of a nerve, and d₁, d₂, d₃, and        d₄ denote the distance between the nerve point and stimulation        electrodes 1-4 (north, east, south and west) respectively, it        can be shown that:

$y = {{\frac{d_{1}^{2} - d_{3}^{2}}{{- 4}R}\mspace{14mu} {and}\mspace{14mu} x} = \frac{d_{2}^{2} - d_{4}^{2}}{{- 4}R}}$

-   -   Where R is the cannula radius, standardized to 1, since angles        and not absolute values are measured.

After conversion from Cartesian coordinates (x,y) to polar coordinates(r,θ), then θ is the angular direction to the nerve. This angulardirection may then be displayed to the user, by way of example only, asthe arrow 90 shown in FIG. 21 pointing towards the nerve. In thisfashion, the surgeon can actively avoid the nerve, thereby increasingpatient safety while accessing the surgical target site. The surgeon mayselect any one of the 4 channels available to perform the DirectionFunction, although the channel with the lowest stimulation currentthreshold (that indicates a nerve closest to the instrument) shouldprobably be used. The surgeon should preferably not move or rotate theinstrument while using the Direction Function, but rather should returnto the Detection Function to continue advancing the instrument.

After establishing an operative corridor to a surgical target site viathe surgical access system 34, any number of suitable instruments and/orimplants may be introduced into the surgical target site depending uponthe particular type of surgery and surgical need. By way of exampleonly, in spinal applications, any number of implants and/or instrumentsmay be introduced through the working cannula 50, including but notlimited to spinal fusion constructs (such as allograft implants, ceramicimplants, cages, mesh, etc.), fixation devices (such as pedicle and/orfacet screws and related tension bands or rod systems), and any numberof motion-preserving devices (including but not limited to total discreplacement systems).

Segregating the Geometric and Electrical Direction Algorithm Models

There are other relationships resulting from the symmetry of the fourelectrodes described above:

d _(w) ² +d _(e) ² =d _(s) ² +d _(n) ²  (1)

and

d ₀ ² +R ²=¼(d _(w) ² +d _(e) ² +d _(s) ² +d _(n) ²)  (2)

where do is the distance between the nerve activation site and themidpoint between the electrodes (i.e., the origin (0, 0) or “virtualcenter”). These results are based purely on geometry and applyindependent of an electrical model.

As described above under the “arc” method, the geometric model can beextended to define a region of uncertainty based on the uncertainty inthe distance to the nerve:

$\begin{matrix}{{x_{\min} = {\frac{1}{4R}\left( {d_{w,\min}^{2} - d_{e,\max}^{2}} \right)}}{x_{\max} = {\frac{1}{4R}\left( {d_{w,\max}^{2} - d_{e,\min}^{2}} \right)}}{y_{\min} = {\frac{1}{4R}\left( {d_{s,\min}^{2} - d_{n,\max}^{2}} \right)}}{y_{\max} = {\frac{1}{4R}\left( {d_{s,\max}^{2} - d_{n,\min}^{2}} \right)}}} & (3)\end{matrix}$

In FIGS. 8 and 13, the two-dimensional x-y model assumes that the nerveactivation site lies in the same plane as the stimulation electrodes orthe entire z-axis space may be considered to be projected onto the x-yplane. It has been found that the z-dimension has no effect on direction“in the plane.” The distance equations presented in the previoussections also apply when the nerve activation site is out of the planeof the stimulation electrodes.

Generalized 1-D Model

FIG. 22 illustrates two electrodes 2200A, 2200B. Given any twoelectrodes 2200A, 2200B, the absolute position of the nerve activationsite 2202 in one dimension can be computed from the distances from thosetwo electrodes:

$\begin{matrix}{d = {\frac{1}{4D}\left( {d_{1}^{2} - d_{2}^{2}} \right)}} & (4)\end{matrix}$

The 1-D model can be extended to two or three dimensions by the additionof electrodes.

3-D Geometric Model

Using the four co-planar electrodes (FIGS. 8 and 13), it is notparticularly easy to identify direction to the nerve along the z-axis.This may be easily rectified by the addition of one or more stimulationelectrodes 2300 (FIG. 23) out of the original x-y electrode plane. Forexample, FIG. 23 shows a k-wire electrode 2300, positioned along thex=y=0 z-axis. D is half the distance between the K-wire electrode 2300and the plane 2302 of the other four electrodes.

3-D direction to the nerve is possible by comparing the distance to thenerve activation site from the k-wire electrode 2300 to that of theother electrodes.

$\begin{matrix}{z = {\frac{1}{4D}\left( {d_{o}^{2} - d_{k}^{2}} \right)}} & (5)\end{matrix}$

where d_(o) (as noted above) is the distance between the nerveactivation site and the midpoint between the electrodes (i.e., theorigin (0, 0) or “virtual center”), and d_(k) is the distance betweenthe nerve activation site and the k-wire electrode 2300. 3-D directionis possible by converting from Cartesian (x, y, z) to spherical (ρ, θ,φ) coordinates. The arc method described above may also be extended tothree dimensions. Other 3-D geometric models may be constructed. Onepossibility is to retain the four planar electrodes 1402A-1402D and adda fifth electrode 1404 along the side of the cannula 1400, as shown inFIG. 14A.

Another possibility is to replace the four planar electrodes 2502A-2502Dwith two pairs (e.g., vertices of a tetrahedron), as shown in FIG. 25.FIG. 25 illustrates a device 2500 with four electrodes 2502A-2502D in atetrahedron configuration, which may be used with the system 20. Fourelectrodes may be a minimum for spanning a 3-D space, and may be themost efficient in terms of number of stimulations required to find thestimulation current thresholds.

Electric Model

The direction algorithm described above assumes direct proportionalitybetween distance and the stimulation current threshold, as in equation(2).

i _(th) =Kd  (6)

where i_(th) is the threshold current, K is a proportionality constantdenoting a relationship between current and distance, and d is thedistance between an electrode and a nerve.

An alternative model expects the stimulation current threshold toincrease with the square of distance:

i _(th) =i _(o) Kd ²  (7)

Using the distance-squared model, the Cartesian coordinates for thenerve activation site can be derived from equations (5), (7) and thefollowing:

$\begin{matrix}{{x = {\frac{1}{4R}\left( {d_{w}^{2} - d_{e}^{2}} \right)}}{y = {\frac{1}{4R}\left( {d_{s}^{2} - d_{n}^{2}} \right)}}{x = {\frac{1}{4{RK}}\left( {i_{w} - i_{e}} \right)}}{y = {\frac{1}{4{RK}}\left( {i_{s} - i_{n}} \right)}}{z = {\frac{1}{4{DK}}\left( {i_{c} - i_{k}} \right)}}} & (8)\end{matrix}$

where i_(x) is the stimulation current threshold of the correspondingstimulation electrode (west, east, south or north), i_(k) is thestimulation current threshold of the k-wire electrode 2602A (FIG. 26),and i_(c) is calculated from:

i _(c) +KR ²=¼(i _(w) +i _(e) +i _(s) +i _(n))  (9)

FIG. 26 illustrates a device 2600, such as a cannula 48A in FIG. 16, anda K-wire 46 slidably received in the device 2600. Both the K-wire 46 andthe device 2600 have electrodes 2602A-2602F.

Other sets of equations may be similarly derived for alternativeelectrode geometries. Note that in each case, i₀ is eliminated from thecalculations. This suggests that the absolute position of the nerveactivation site relative to the stimulation electrodes may be calculatedknowing only K. As noted above, K is a proportionality constant denotingthe relationship between current and distance.

Distance or position of the neural tissue may be determined independentof nerve status or pathology (i.e., elevated i₀), so long as stimulationcurrent thresholds can be found for each electrode.

Measuring Nerve Pathology

If the distance to the nerve is known (perhaps through the methodsdescribed above), then it is possible to solve equation (8) for i₀. Thiswould permit detection of nerves with elevated stimulation thresholds,which may provide useful clinical (nerve pathology) information.

i _(o) =i _(th) −Kd ²  (10)

Removing Dependence on K

The preceding descriptions assume that the value for K is known. It isalso possible to measure distance to a nerve activation site withoutknowing K, by performing the same measurement from two differentelectrode sets. FIG. 24 illustrates two pairs of electrodes 2400A,2400B, 2402A, 2402B and a nerve activation site 2404. The top twoelectrodes 2400A, 2400B form one pair, and the bottom two electrodes2402A, 2402B form a second pair. Using the electrodes 2400A, 2400B,2402A, 2402B and distances defined in FIG. 24, the geometric model fromequation (4) becomes:

$\begin{matrix}{\frac{d_{a}}{d_{b}} = {{\frac{D_{b}}{D_{a}}\left( \frac{d_{3}^{2} - d_{4}^{2}}{d_{1}^{2} - d_{2}^{2}} \right)} = {\frac{d_{0} + d_{b}}{d_{b}} = {\frac{d_{0}}{d_{b}} + 1}}}} & (11)\end{matrix}$

Adding the electrical model from equation (7), the dependence on K isremoved. Solve equation (11) for d_(b) to get the distance in onedimension:

$\begin{matrix}{{\frac{D_{b}}{D_{a}}\left( \frac{i_{3} - i_{4}}{i_{1} - i_{2}} \right)} = {\frac{d_{0}}{d_{b}} + 1}} & (12)\end{matrix}$

Finally, it is possible to solve for the value of K itself:

$\begin{matrix}{K = \frac{i_{1} - i_{2}}{4D_{b}d_{b}}} & (13)\end{matrix}$

Although the configuration in FIG. 14 shows four electrodes, thetechnique may also work with three collinear electrodes.

Electrode Redundancy

Whichever electrical model is used, the relationship expressed inequation (1) means that the current at any of the electrodes at the fourcompass points can be “predicted” from the current values of the otherthree electrodes. Using the electrical model of equation (7) yields:

i _(w) ² +i _(e) ² =i _(s) ² −i _(n) ²

Using the electrical model of equation (6) yields:

i _(w) +i _(e) =i _(s) +i _(n)

This provides a simple means to validate either electrical model.

Applying the tools of geometric and electrical modeling may help tocreate more efficient, accurate measurements of the nerve location.

While certain embodiments have been described, it will be appreciated bythose skilled in the art that variations may be accomplished in view ofthese teachings without deviating from the spirit or scope of thepresent application. For example, the system 22 may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory act to practicing the system 20 or constructing anapparatus according to the application, the computer programming code(whether software or firmware) according to the application willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as ROMs, PROMs, etc., thereby making anarticle of manufacture in accordance with the application. The articleof manufacture containing the computer programming code may be used byeither executing the code directly from the storage device, by copyingthe code from the storage device into another storage device such as ahard disk, RAM, etc. or by transmitting the code on a network for remoteexecution. As can be envisioned by one of skill in the art, manydifferent combinations of the above may be used and accordingly thepresent application is not limited by the scope of the appended claims.

What is claimed is:
 1. A system for finding a direction of a nerve froma surgical instrument comprising: a surgical accessory having at least afirst stimulation electrode on a distal region and a second stimulationelectrode spaced apart from the first stimulation electrode, thestimulation electrodes each configured to deliver an electricalstimulation signal; at least one sensor configured to detect an evokedresponse from a nerve depolarized by the electrical stimulation signals;and a control unit in communication with the surgical accessory and theat least one sensor, the control unit being configured to (a)electrically stimulate the first stimulation electrodes with a first setof at least one stimulation signal until a first initial bracket withinwhich a first threshold stimulation current level must lie isdetermined, (b) electrically stimulate the second stimulation electrodeuntil a second initial bracket within which a second thresholdstimulation current level must lie is determined, (c) process thedetermined first and second initial brackets to find a direction fromthe surgical accessory to the nerve, said processing includingdetermining the first threshold stimulation current level by narrowingthe first initial bracket to a first final bracket and determining thesecond threshold stimulation current level by narrowing the secondinitial bracket to a second final bracket, (d) display on a displaycommunicatively linked to the control unit, an initial indicatorindicating a general direction of the nerve from the surgical accessory,the general direction based on the determination of said first andsecond initial brackets, and (e) display on the display, a subsequentindicator indicating a more specific direction of the nerve from basedon the determination of first and second threshold stimulation currentlevels.
 2. The system of claim 1, wherein the surgical accessory isconfigured to advance through tissue to create an operative corridor toa surgical target site.
 3. The system of claim 1, wherein the surgicalaccessory comprises at least one of a series of sequential dilationcannulae, and a K-wire.
 4. The system of claim 1, wherein the K-wire isslidably received in the surgical accessory.
 5. The system of claim 2,wherein the system for establishing an operative corridor to thesurgical target site is configured to access a spinal target site. 6.The system of claim 5, wherein the system for establishing an operativecorridor to the surgical target site is configured to establish theoperative corridor via a lateral, trans-psoas approach.
 7. The system ofclaim 1, further comprising a handle coupled to the surgical accessory,the handle having at least one button for initiating the electricalstimulation button from the control unit to the first and secondstimulation electrodes on said surgical accessory.
 8. The system ofclaim 1, wherein the display is further operable to display anelectromyographic (EMG) response of the muscle.
 9. The system of claim1, wherein the display comprises a touch-screen display operable toreceive commands from a user.
 10. The system of claim 1, wherein thesurgical accessory further comprises third and fourth stimulationelectrodes, each of the first, second, third and fourth electrodes beingspaced apart from one another.
 11. The system of claim 14, wherein thestimulation electrodes are positioned in a two-dimensional plane. 12.The system of claim 14, wherein the stimulation electrodes arepositioned orthogonally to form a cross.
 13. The system of claim 12,wherein the control unit is configured to derive x and y Cartesiancoordinates of a nerve direction with respect to the surgical accessoryby using x=i_(w) ²−i_(e) ² and y=i_(s) ²−i_(n) ² where i_(e), i_(w),i_(n), and i_(s) represent threshold stimulation current levels forfirst, second, third, and fourth electrodes.
 14. The system of claim 10,wherein the stimulation electrodes comprise a first set of electrodes ina first two-dimensional plane and a second set of at least one electrodein another plane that is parallel to the first plane.
 15. The system ofclaim 14, wherein the stimulation electrodes form a tetrahedron.
 16. Thesystem of claim 14, wherein the control unit is configured to determinea three-dimensional vector from a reference point on the surgicalaccessory to the nerve.
 17. The system of claim 16, wherein the controlunit is configured to determine the three-dimensional vector from areference point on the surgical accessory to a nerve by using:${x = {\frac{1}{4{RK}}\left( {i_{w} - i_{e}} \right)}}\;$$y = {{\frac{1}{4{RK}}\left( {i_{s} - i_{n}} \right)\mspace{14mu} {and}\mspace{14mu} z} = {\frac{1}{4{DK}}\left( {i_{c} - i_{k}} \right)}}$where i_(x) is a threshold stimulation current level of a correspondingstimulation electrode (first, second, third, fourth), i_(k) is thestimulation current threshold of a k-wire electrode, and i_(c) iscalculated from:i _(c) +KR ²=¼(i _(w) +i _(e) +i _(s) +i _(n)).
 18. The system of claim16, wherein the control unit is further configured to display thethree-dimensional vector to a user.
 19. The system of claim 10, whereinthe stimulation electrodes comprise two pairs of electrodes.
 20. Thesystem of claim 10, wherein the control unit is configured toelectrically stimulate the first stimulation electrode with a firstelectrical stimulation signal, determine whether the first thresholdstimulation current level has been bracketed, stimulate the secondstimulation electrode with a second electrical stimulation signal anddetermine whether the second threshold stimulation current level hasbeen bracketed.
 21. The system of claim 20, wherein the first and secondelectrical stimulation signals are equal.
 22. The system of claim 20,wherein the control unit is further configured to determine a firstrange for the first threshold stimulation current level, and determine asecond range for the second threshold stimulation current level, eachhaving a maximum stimulation current threshold value and a minimumstimulation current threshold value.
 23. The system of claim 22, whereinthe control unit is configured to process the first and second ranges byusing x_(min)=i_(w,min) ²−i_(e,max) ²; x_(max)=i_(w,max) ²−i_(e,min) ²;y_(min)=i_(s,min) ²−i_(n,max) ²; y_(max)=i_(s,max) ²−i_(n,min) ², wherei_(e), i_(w), i_(n), and i_(s) represent threshold stimulation currentlevels for first, second, third, and fourth electrodes.
 24. The systemof claim 1, wherein the initial indicator comprises an arc.
 25. Thesystem of claim 1, wherein the subsequent indicator comprises one of anarrowed arc and an arrow.
 26. The system of claim 1, wherein thecontrol unit is further configured to electrically stimulate the firstand second stimulation electrodes to bisect each of the first and secondinitial brackets until a first stimulation current threshold level hasbeen found for the first stimulation electrode and a second stimulationcurrent threshold level has been found for the second stimulationelectrode within a predetermined range of accuracy.
 27. The system ofclaim 26, wherein the control unit is configured to narrow the arcindicating a general direction of the nerve from the surgical accessoryas the first and second initial brackets are bisected.
 28. The system ofclaim 1, wherein the control unit is configured to emit a sound when thecontrol unit determines a distance between the surgical accessory hasreached a predetermined level.
 29. The system of claim 1, wherein thecontrol unit is configured to emit a sound that indicates a distancebetween the surgical accessory and the nerve.
 30. The system of claim 1,wherein the surgical accessory is dimensioned to be insertedpercutaneously through a hole to a surgical site.
 31. The system ofclaim 1, wherein the control unit is further configured to determine thefirst initial bracket and second initial bracket by detecting a responseof the nerve depolarized by the first set of at least one stimulationsignal and the second set of at least one stimulation signal.
 32. Thesystem of claim 10, wherein the control unit is further configured to(f) electrically stimulate the third stimulation electrode with a thirdset of at least one stimulation signal until a third initial bracketwithin which a third threshold stimulation current level must lie isdetermined, (g) electrically stimulate the fourth stimulation electrodeuntil a second initial bracket within which a second thresholdstimulation current level must lie is determined, (h) process thedetermined third and fourth initial brackets to find a direction fromthe surgical accessory to the nerve, said processing includingdetermining the third threshold stimulation current level by narrowingthe third initial bracket to a third final bracket and determining thefourth threshold stimulation current level by narrowing the fourthinitial bracket to a fourth final bracket, (i) display on a displaycommunicatively linked to the control unit, an initial indicatorindicating a general direction of the nerve from the surgical accessory,the general direction based on the determination of said third andfourth initial brackets, and (j) display on the display, a subsequentindicator indicating a more specific direction of the nerve from basedon the determination of third and fourth threshold stimulation currentlevel.
 33. The system of claim 32, wherein the control unit is furtherconfigured to electrically stimulate the third and fourth stimulationelectrodes to bisect each of the third and fourth initial brackets untila third stimulation current threshold level has been found for the thirdstimulation electrode and a fourth stimulation current threshold levelhas been found for the fourth stimulation electrode within apredetermined range of accuracy.
 34. The system of claim 33, wherein thecontrol unit is configured successively narrow the arc indicating ageneral direction of a nerve from the surgical accessory as the first,second, third, and fourth initial brackets are bisected.
 35. The systemof claim 34, wherein the control unit is further configured toelectrically stimulate the first, second, third, and fourth stimulationelectrodes to bisect each of the first, second, third, and fourthstimulation electrodes to bisect each of the first initial bracket,second initial bracket, third initial bracket, and fourth initialbracket until the first threshold stimulation current level has beenfound for the first stimulation electrode, the second thresholdstimulation current level has been found for the second stimulationelectrode, the third threshold stimulation current level has been foundfor the third stimulation electrode, the fourth threshold stimulationcurrent level has been found for the fourth stimulation electrode withina predetermined range of accuracy.
 36. The system of claim 35, whereinthe control unit is configured to narrow the arc indicating a generaldirection of a nerve from the surgical accessory as the first, second,third, and fourth initial brackets are bisected.
 37. The system of claim1, wherein the first set of at least one stimulation signal comprisesstimulation signals of sequentially doubled current intensity.